During piercing, drilling, and cutting in metal workpiece materials with a laser beam, in particular when the processing operation is supported by oxygen as the process gas, spontaneous, generally undesirable material removal processes may occur. During piercing, the spontaneous material removal may be so intense that unacceptably large quantities of particles are removed. The spontaneous material removal may lead to contamination, damage or even destruction of laser processing machines and the optical elements close to the process and of the processing nozzles.
JP2003251477A describes a laser processing device and a laser processing method for producing a hole to identify an error directly after an incorrect hole has been produced. To this end, a portion of the pulsed processing laser beam is branched off. The pulsed energy or pulsed power of the branched part-beam is integrated and the integrated value is compared with a predetermined acceptable energy level which is required to produce the hole.
JP8010976A describes a method for piercing a workpiece in which the intensity of radiation reflected on the workpiece is detected and compared with a threshold value in order to decide whether the piercing operation is ended.
It is also known from DE3926859A1, DE4336136A1, EPO470583B1, JP06091384A, and JP06246466A to detect the occurrence of spontaneous material removal processes (“blow up”) and/or the ending of the piercing operation by measuring the (process) radiation emitted from the workpiece. In these documents, it is proposed to detect the occurrence of an undesirable process state by comparing the continuously detected quantity of light with a previously determined reference or limit value. When the limit value is exceeded, the processing parameters can be changed by controlling the processing machine in order to prevent the spontaneous material removal. When the limit value is significantly exceeded, the processing operation is stopped.
It is desirable to keep the time required for the piercing or drilling of a hole as short as possible to increase the productivity of the laser processing machine. However, this time cannot be shortened at will without significantly reducing the quality of the perforated hole since, in the event of an excessive application of energy, the above-described spontaneous material removal processes can occur.